Allow customization of Spring Integration objects used in Spring Cloud Stream

[elefeint]

We don't seem to give Spring Cloud Stream client apps much ability to configure PubSubInboundChannelAdapter or PubSubMessageSource, since PubSubMessageChannelBinder directly instantiates both in its own code.

Both PubSubInboundChannelAdapter and PubSubMessageSource would benefit from ability to customize headerMapper and payloadType.

PubSubMessageSource also needs to allow customization of maxFetchSize -- in Spring Cloud Stream integration, it would give better performance to pull a batch over the network, and then stream it internally within the app.

The simplest way to implement the properties is by adding them to PubSubConsumerProperties.

[artembilan]

Note: the payloadType for Spring Cloud Stream must always be byte[]. The Spring Cloud Stream is responsible for proper conversion from byte[].

The headerMapper cannot be configured via properties since it has to be a bean.
We could provide some default one and allow end-user to override it with his/her own preferences. But again: do we really need to do that, unless just map everything we receive from Pub/Sub.
We talk about Spring Cloud Stream which has its own strong opinion.

We definitely can provide a hook to configure a maxFetchSize though.

Need also to dig for more properties if that...

[elefeint]

Ah, so only maxFetchSize is reasonable. That should be easy then!